Stationary gas turbines and aircraft turbines having rotors composed of a plurality of rotor disks are generally known. One central tie rod or a plurality of eccentric tie rods clamp the rotor disks together. For this purpose, the rotor disks have at least one cylindrical bore through which the tie rods extend.
Rotor disks of this type are known, for example, from U.S. Pat. No. 2,579,745. Each rotor disk is I-shaped in cross section and bears the rotor blades of the turbine or compressor on its outer flange, which is arranged parallel to the axis of rotation. The radially inner flange likewise extends parallel to the axis of rotation, radially inwardly directed projections being provided at the outer ends, as seen in the axial direction, of the inner flange. As a result, the inner flange of the rotor disk has a recess located between the projections, and the circumferential surface, facing the axis of rotation of the rotor, of this recess has a cylindrical profile in the central region between the two outer projections.
Moreover, GB 219 655 has disclosed a rotor disk with a central bore, at which a sprung arm which projects freely on one side is provided on the hub side, as seen in the axial direction. To improve the spring action of the arm, the latter is tapered in the central region of its axial extent.
Furthermore, JP 62-251403 A has disclosed a single-piece rotor for a twin-flow steam turbine, this rotor having a central bore. To reduce the density of material stresses in the tangential direction, the central rotor bore has a recess which is annular in cross section, runs around the inner circumference and lies approximately parallel to the reference stress lines.
On its outer circumference, each rotor disk bears rotor blades which are arranged in a ring and around which a flow medium can flow in order for said flow medium to be compressed or for rotational energy to be absorbed from a flow medium. In operation, the rotor blades secured to the rotor disk produce huge centrifugal forces, and consequently each rotor disk is exposed to high levels of load.
The rotor disks must be entirely free of defects if they are to be able to withstand these loads. To ensure that this is the case, it is known to use suitable test methods which can be used to examine the rotor disk for cracks and defects prior to initial use and also during repeat tests, in order to ensure a minimum service life and therefore safe operation of the turbo machine.
The ability to detect cracks during the tests is increasingly restricted by the increasing size of rotor disks with a bore or if coarse-grain materials are used.
One way of ensuring the required service life is the deliberate introduction of compressive residual stresses into the material of the rotor disks, which delay the growth of defects, i.e. cracks, during subsequent operation. For this purpose, while the rotor disk with a bore is being produced, it is deliberately overloaded, i.e. it is spun at a rotational speed which is higher than the nominal rotational speed of the rotor. This causes plastic deformation in the region of the bore, leading to compressive residual stresses. However, the level of the compressive residual stresses in the disk material is limited by the maximum spinning speed of the spinning test bench and by the temperature during spinning, and consequently fewer compressive residual stresses can be produced than would ultimately be desirable.
The defects in the rotor disk which have not been detected and/or cannot be tolerated may continue to produce and enlarge cracks, on account of the high levels of load and the limited level of compressive residual stresses, and these cracks reduce the service life of the rotor disk and therefore of the turbo machine.